In the conventional art, a multi-piece wiring substrate is known in which a plurality of wiring substrate regions are arranged in a matrix substrate in longitudinal and transverse directions. Then, dividing grooves are formed in principal faces such as an upper surface of the matrix substrate along boundaries of each wiring substrate region. When a bending stress is applied to the matrix substrate on both sides of each dividing groove, the matrix substrate is fractured and divided into individual pieces of wiring substrates. For example, the dividing grooves are formed by forming a cut in a predetermined depth by using a cutter blade or the like along an outer periphery of each wiring substrate region in the upper surface and the lower surface of the matrix substrate not yet fired. Further, in a case where a plurality of through holes are formed across the outer periphery of the wiring substrate region, by virtue of these through holes and the dividing grooves described above, the matrix substrate is relatively easily divided into individual pieces of wiring substrates. Here, in a case where an inner surface conductor is provided on the inner wall of the through hole, such a configuration can be achieved that the wiring conductors of adjacent wiring substrate regions are electrically connected together.
In recent years, individual wiring substrates are reduced in size. In association with this size reduction of wiring substrates, the width of each frame is also remarkably reduced, so that a thickness of a portion (the frame) for separating the mounting section from an external environment is remarkably reduced. In a multi-piece wiring substrate in which such wiring substrate regions whose size reduction is remarkable are arranged, for a reason of precision in mechanical processing or the like, it is difficult to provide a dividing groove along the boundary of adjacent wiring substrate regions by using a cutter blade. Thus, there is proposed a method of forming the dividing grooves by using a laser beam having excellent positional precision (see Japanese Unexamined Patent Publication JP-A 2012-81647).
Further, in this multi-piece wiring substrate, in some cases, the through holes are not formed in the outer periphery of the wiring substrate region. As such, in a case where inner conductors are provided and through holes (inner surface conductors) are not formed, in order to electrically connect together the wiring conductors of adjacent wiring substrate regions, wiring conductors for connection are provided in an inner layer in which the dividing grooves are not formed, in a thickness direction of the matrix substrate (see Japanese Unexamined Patent Publication JP-A 2007-318034).